Previously reported examples of methods for producing unsaturated carboxylic acid-modified vinyl alcohol polymers (b) consist mainly of [1] a method comprising reacting (meth) acrylic acid chloride and polyvinyl alcohol in an organic solvent (see Patent Document 1), and [2] a method comprising reacting (meth)acrylic acid anhydride and polyvinyl alcohol in an organic solvent (see Patent Document 2).
In the above-mentioned methods of [1] and [2], a chloride or anhydride of (meth) acrylic acid is used as an esterification agent to enhance the reactivity of polyvinyl alcohol. However, chlorides and anhydrides of (meth) acrylic acid are expensive and not readily available industrially. Moreover, since a reprecipitation procedure using an organic solvent as a poor solvent is carried out for purification, these production methods cannot be said to be industrially advantageous methods.
Consequently, studies, although extremely few in number, have been conducted on the synthesis of unsaturated carboxylic acid-modified vinyl alcohol polymers using industrially inexpensive and readily available unsaturated carboxylic acids as raw materials, a known example of which consists of [3] a method comprising reacting (meth)acrylic acid with polyvinyl alcohol in aqueous hydrochloric acid (see Non-Patent Document 1). However, since the method of [3] uses dialysis for purification, it is not an industrially advantageous production method.
In addition, in addition to the method of [3] above, another known method consists of [4] reacting polyvinyl alcohol, acetic acid and (meth)acrylic acid using hydrochloric acid as an acid catalyst (see Non-Patent Document 2). However, the method of [4] requires the addition of acetic acid, and the resulting modified vinyl alcohol polymer contains not only a (meth)acrylic acid of an unsaturated carboxylic acid compound, but also an acetic acid of a saturated carboxylic acid compound. Moreover, since purification is carried out by a reprecipitation procedure using an organic solvent for the poor solvent, this is not an industrially advantageous production method.
Accordingly, a sufficiently industrially advantageous method for producing unsaturated carboxylic acid-modified vinyl alcohol polymers has yet to be known.
Moreover, a gas barrier film using an unsaturated carboxylic acid-modified vinyl alcohol polymer obtained according to the methods described above is also not known.
[Patent Document 1] US Patent No. 5373034
[Patent Document 2] German laid-Open Patent No. 3322993
[Non-Patent Document 1] Angewandte Maskromolekulare Chemie, pp. 179-202, vol. 113, 1983
[Non-Patent Document 2] Journal of Polymer Sciences A: Polymer Chemistry, pp. 3603-3611, Vol. 35, 1997